Warped Passages - Lisa Randall [78]
Not all QED processes involve a photon that is destroyed, however. In addition to the ephemeral intermediate or internal particles*—like the photons leading to electromagnetic interactions that are produced and almost immediately destroyed—there are also real, external photons, particles that enter or leave an interaction region. Sometimes those particles are deflected and sometimes they turn into other particles. Either way, the particles that enter or leave are real physical particles.
Quantum Field Theory
Quantum field theory, the tool with which we study particles,* is based on eternal, omnipresent objects that can create and destroy those particles. These objects are the “fields” of quantum field theory. Like the classical electromagnetic fields that inspired their name, quantum fields are objects that permeate spacetime. But quantum fields play a different role. They create or absorb elementary particles. According to quantum field theory, particles can be produced or destroyed anywhere and at any time.
For example, an electron or a photon can appear or disappear anywhere in space. Quantum processes allow the number of charged particles in the universe to change through particle creation or destruction. Each particle is created or destroyed by its own particular field. In quantum field theory, not only electromagnetism but all forces and interactions are described in terms of fields, which can create new particles or eliminate particles that were already present.
According to quantum field theory, you can think of particles as excitations of the quantum field. Whereas the vacuum, a state with no particles, contains only constant fields, states with particles present contain fields with bumps and wiggles corresponding to the particles. When the field acquires a bump, a particle is created, and when it absorbs this bump to become constant once again, the particle is destroyed.
The fields that create electrons and photons must exist everywhere to guarantee that all interactions can occur at any point in spacetime. This is essential because interactions are local, which is to say that only particles in the same place can participate. Action at a distance would be more like magic. Particles don’t have ESP—they have to be in contact to interact directly.
Electromagnetic interactions do occur between distant charges that are not in direct contact, but only through the auspices of the photon or some other particle that has direct contact with both of the interacting charged particles. In that case, charges appear to affect each other instantaneously, but only because the speed of light is so fast. Really, the interaction only occurred through local processes; the photon first coincided with one of the charged particles and then the other. The field therefore had to create and destroy the photon at the precise locations of the charged particles.
Antiparticles and the Positron
Quantum field theory also tells us that for each particle, a counterpart must exist, known as an antiparticle. Tom Stoppard talks about antiparticles in his play Hapgood: “When a particle meets an anti-particle, they annihilate each other, they turn into energy-bang, you understand.” Any science fiction fan knows about antiparticles—they are what you make guns from to destroy the universe and are also what powers Star Trek’s USS Enterprise.
Those last applications are fictitious, but antiparticles are not. Antiparticles are truly a part of the particle physics view of the world. In field theory and the Standard Model, they are as essential as particles. In fact, antiparticles are just like particles, except that all their charges are opposite.
Paul Dirac first encountered antiparticles when he developed the quantum field theory describing the electron. He found that a quantum field theory that is consistent with both quantum mechanics and special relativity necessarily includes antiparticles. He hadn’t deliberately